When lawmakers want to illustrate their points on the floor of the House or the Senate, they often use visual aids - big, brightly colored poster boards, known as floor charts. Well, NPR's Tamara Keith is one of our congressional correspondents, so she sees these posters all the time. And she set out to learn how and why they've become an essential part of congressional messaging.

TAMARA KEITH, BYLINE: Almost every day, the House is in session. Representatives line up to give what are known as one-minute speeches. Florida Democrat Frederica Wilson is always there.

REPRESENTATIVE JOHN BOEHNER: For what purpose does the gentlewoman from Florida seek recognition?

KEITH: And she always has her floor chart with her.

REPRESENTATIVE FREDERICA WILSON: Mr. Speaker, it's now been 885 days since I arrived...

KEITH: It lists the number of days since Wilson came to Congress and the number of Americans unemployed. There are stock photos of a plumber, a college graduate and a medical professional.

WILSON: When you are in the minority, you have to find ways to get your message across because you don't - there's no other way. There's no committee that will receive your suggestions.

KEITH: She's been reusing the same chart since February, just swapping out the number of days in red type. Some members have dozens of them, ready to go at a moment's notice. Indiana Republican Congressman Todd Rokita has a whole stack of charts in his office, leftover from a lengthy presentation he gave back in April.

REPRESENTATIVE TODD ROKITA: Mr. Speaker, I rise today to talk about, really, the most important issue of our time in this country and, really, the world.

KEITH: The national debt.

ROKITA: This isn't a partisan set of remarks because it's not a partisan issue. In fact, it's very bipartisan, and this chart here shows that.

KEITH: It's a bar chart showing budget deficits through the years with pictures of presidents on top of each bar. If you'd seen the speech on C-SPAN, occasionally you'd see a hand come into the shot, switching to the next chart. That is Zach Zagar, Rokita's communications director.

ZACH ZAGAR: I was Vanna White on the House floor one beautiful evening this spring.

KEITH: I visited his office to get an up-close look at these charts to figure out how they're made.

ZAGAR: And...

KEITH: First, the content. These are actually just PowerPoint slides from a presentation Rokita often gives when he's back in his home district, printed real big for use on the House floor. There are a couple of nice ones, made expertly and mounted by the House graphics office. But most in this stack are just printed on giant sheets of paper, then wrapped around and taped onto previously used poster boards. Zagar says the House Republican Conference has a big printer, which makes these charts cheap to make, if not aesthetically perfect close up.

ZAGAR: Sometimes, you get the back end of a weird leftover presentation. Sometimes, you get a piece of poster board with a giant wedge taken out of it, so, yeah, it varies. The presentation on - via television is barely noticeable.

KEITH: A little secret about Congress that may not be obvious watching on TV: Often, when members give these speeches, the chamber is virtually empty. But that doesn't really matter because C-SPAN's cameras are always rolling. Bill Gray is a producer at C-SPAN, and a man so obsessed with floor charts he's created a blog to catalogue their use.

BILL GRAY: Budget and deficit and deficit reduction and anything that has to do with hard numbers, those are the most popular because if you show a giant red line going from low to high, then it's going to draw the eye, and it's also just very simple. This number is bigger than it used to be. There we go.

KEITH: But perhaps the most popular floor chart of all time - though, admittedly this is hard to gauge - was used by Senator Chuck Grassley back in 2009.

(SOUNDBITE OF ARCHIVED AUDIO)

SENATOR CHUCK GRASSLEY: As you can see here, you know, we have the debt and deficit dragon.

KEITH: You heard that right. The debt and deficit dragon, a gray fire-breathing dragon labeled with yellow old English-style print on a blue background. It got a lot of attention, which is exactly what Grassley says he's going for.

GRASSLEY: I think they're very beneficial, probably more to the public at large than they are to our colleagues.

KEITH: At this point, a taxpayer might wonder how much these charts cost.

In reality, it varies, from an estimated $10 for the giant-printer-used-poster-board method to, well, no one would say how much it costs to get one of the fancy charts made by the House and Senate graphics offices. Something comparable made by a national printing chain would cost $129 per chart. But everyone insists they aren't spending that much. Tamara Keith, NPR News, the Capitol. Transcript provided by NPR, Copyright NPR.